Metal oxide surfaces, growth kinetics

Growth Kinetics of Phosphate Fihns on Metal Oxide Surfaces... 

Oxide surfaces, and in particular oxide films, are versatile substrates for the preparation of model catalysts. Quite a few of these systems show nanoscale reconstructions, which can be employed as templates for the growth of ordered model catalysts of reduced complexity. In order to efficiently control the growth of nanostructured metal particle arrays, two conditions have to be met. First, the template must provide sites of high interaction energy that trap the deposited metals. Second, the kinetics of the growth process must be carefully controlled by choosing... 

Supported model catalysts are frequently prepared by thermally evaporating metal atoms onto a planar oxide surface in UHV. The morphology and growth of supported metal clusters depend on a number of factors such as substrate morphology, the deposition rate, and the surface temperature. For a controlled synthesis of supported model catalysts, it is necessary to monitor the growth kinetics of supported metal... 

Metal oxidation is a heterogeneous solid state reaction and starts in the same way as other heterogeneous reactions with nucleation and initial growth. This was discussed in Chapter 6. A time-dependent nucleation rate may dominate the overall growth kinetics of thin Films. Even under an optical microscope (i.e., in macroscopic dimensions), preferential sites of growth can still be discerned [J. Benard (1971)). This indicates that lateral transport on the surface (e.g., at sites where screw dislocations emerge) can possibly be more important for the initial reactive growth than transport across thin oxide layers. 

The first effect describes the dependence of the hydroxylation degree on the temperature hence, it determines the temperature dependence of the film growth rate. The second effect restricts the maximum coverage of the oxide surface by metallic precursors and determines the maximum growth rate. For the dependence of the kinetic parameters on local environment, we used the following values ... 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Scanning probe microscopies are now able to study in situ the growth of metal clusters. These studies are performed sequentially after deposition. On metal/metal systems it has been possible to follow the nucleation kinetics and to derive the elementary energies like adsorption and diffusion energies (see the excellent review by Brune [68]). On oxide surfaces only recently such studies have been undertaken. STM can be only used on conducting samples, however it is possible to use as a support an ultrathin film of oxide grown on a metal. By this way it has been possible to study the nucleation of several... 

While the thermal oxidation of a compact metal surface is usually limited to the growth of an oxide layer with a thickness of a few of nanometers, bulk metal nanostmctures can be fully converted into the corresponding oxide or chalcogenide. Again the relative diffusion rate of metal atoms and the oxidation agent in the oxide determine the oxidation kinetics and structure formation. A topographic transformation to a metal oxide nanostructure is observed when the mobility of the oxidation agent exceeds the one of the metal atoms. When this is not the case, the so-called nanoscale Kirkendall effect (NKE) responsible for the formation of sophisticated hollow nanostructures, such as nanospheres, nanotubes, and nanopeapods, proceeds [2-5]. 

---